Dual Focusing Mechanism

[B.S.Kumar]

I'm curious about the coarse and fine focusing mechanism found in microscopes, where the outer, larger knob controls coarse focusing and the inner knob controls fine focusing.

What kind of mechanism is used?

Any pointers or links will be greatly appreciated.

Thanks,
Kumar

[ramesesjd]

Hi Kumar,

On the microscopes I've used, turning the outer knob at a given rate causes the inner knob to turns at a faster, and vice versa. Thus, I suspect the outer and inner knobs are connected through a planetary gear arrangement. The rotation is converted into linear motion of the stage or eyepiece/eyepieces (depending on the age/design of the microscope) through a rack-and-pinion arrangement.

I hope that helps. Others here have more experience than I do, so I'm curious to see what they say.

Regards,
-R

[B.S.Kumar]

Ramesesjd,

I was at a lab the other day when I saw this dual focus mechanism. I asked to use it, and was amazed at the smoothness. The inner knob did turn along with the outer knob, but if I held the inner knob, it did not prevent the outer knob from being turned. This makes me believe that the two knobs are not physically connected.

Kumar

[ramesesjd]

Hi Kumar,

That's very interesting. Since the inner knob turned along with the outer knob there surely must be some connection. Perhaps clutch is engaged when the inner knob experiences too much torque (e.g. like being held while the outer knob is turned) to prevent damage. I might be totally off here but I'm now even more curious to hear what others have to say.

Cheers,
-R

[Audii-Dudii]

FYI, Cambo now offers an optional (and expensive!) fine-focus knob for its Actus series cameras: https://www.cambo.com/en/actus-serie...camera/ac-380/



It's claimed to replace the original focusing knob, so whatever magic is involved here is contained within the knob itself and is not part of the original focusing mechanism.

[R.K]

I never see that mechanizm. But if it is a planetary reduser, than it will allow to turn a I high speed wheel when the outer low speed wheel hold unmoved. The speed ot rotation will change, but it may be any way faster then when you turn the outer slow speed wheel.

[Oren Grad]

The LPL 4x5 enlargers have coaxial coarse and fine focus in the same way, though the control on my 4500II doesn't match the precision feel of the best microscopes I used back in the day.

[Jim C.]

Planetary it is -
https://www.quora.com/What-is-coarse...n-a-microscope

[Jim C.]

one more for the mechanically inclined -
https://patentimages.storage.googlea.../US4482221.pdf

[Bernice Loui]

It's complex depending on the specific microscope and the requirements put on it. Stereo microscope (low magnification) tend to have a planetary reduction system often done by tension loaded chrome steel balls with a shaft in the center and a spherical outer housing for the balls to ride on. These planetary reduction systems are run in special friction lubricants that serve to increase friction and reduce wear.

Example of a good microscope focus block design can be found on the Leitz Ortholux_Orthoplan. It is quite a cleaver mechanism that is backlash free with very precise movements, very durable, and stable once set. These are the mechanical demands placed on any good microscope. Description of how it works is on page 15-16 of this document:

http://www.microscopy-uk.org.uk/mag/..._4.pdfypically

Cameras typically do not need this precise and complex of a focus mechanism due to the scale of subjects and magnifications and ... involved.


Bernice